I don't know if I'm doing this correctly or not:

When something like Flechette Torpedoes hit a target, I always use the ship's original Hull value as the measure that determines whether a hit by the torpedoes causes stress.

I use the same measure where Stealth Device and Proton Rockets, which add damage based on agility.

What's the 'actual' rule with each of these?

Hull Value does not technically get reduced by damage. A ship is destroyed when it has damage cards equal to or greater than its hull value, the value does not decrease. Unless there are some weird corner cases that I can't think of right now, a ship will always have the hull value it began the game with (and as far as I know, the only thing that can modify it currently is a hull upgrade).

Like Hull Upgrade, Stealth Device modifies the ship's beginning stats, its agility goes up. Unlike hull Upgrade, Stealth Device actually includes rules for the card, and thus the bonus to be discarded. So yes, SD will increase the dice used for prockets, but only so long as the SD is still on the ship and only within the limits of the Proton Rockets card (so Agility 4 is wasted).

edit: oh, and in case it should come up, the actual FAQ includes at least one reference to "remaining hull points". That is VERY sloppy wording on FFGs part and that entry should be read in the spirit it was intended, not the precise literal meaning.

From FAQ:

Flechette Torpedoes

When determining whether the defender receives a stress token from Flechette Torpedoes, the defender’s starting hull value (including any equipped Hull Upgrade card) is used, not the defender’s remaining hull points.

I don't know if I'm doing this correctly or not:

When something like Flechette Torpedoes hit a target, I always use the ship's original Hull value as the measure that determines whether a hit by the torpedoes causes stress.

I use the same measure where Stealth Device and Proton Rockets, which add damage based on agility.

What's the 'actual' rule with each of these?

Raising your hull with Hull Upgrade makes you immune to Flechettes, although only the HWK, StarViper and Kihraxz can actually do this.

Increasing your agility via card increases the damage of Proton Rockets, although bonus damage caps at 3 and you usually only take Prockets on 3 agility ships anyway.

I don't know if I'm doing this correctly or not:

When something like Flechette Torpedoes hit a target, I always use the ship's original Hull value as the measure that determines whether a hit by the torpedoes causes stress.

I use the same measure where Stealth Device and Proton Rockets, which add damage based on agility.

What's the 'actual' rule with each of these?

Raising your hull with Hull Upgrade makes you immune to Flechettes, although only the HWK, StarViper and Kihraxz can actually do this.

Increasing your agility via card increases the damage of Proton Rockets, although bonus damage caps at 3 and you usually only take Prockets on 3 agility ships anyway.

Don't forget the Aggressor - starting value is 4 hull.

I actually learned something new here. I didn't realize that Ships with hulls of 5 and over were immune. Good to know!

Decimator, Firespray, Shuttle, Tie Bomber, Punisher, YT-1300, YT-2400, Y-wing, Hound's Tooth, and K-wing are all naturally immune.